This invention relates to DC electric motor controls and more particularly to a method and apparatus for sensing plugging of the motor.
In series DC electric motor controls of the prior art various methods have been utilized to determine when the motor is in a dynamic braking or plugging mode in order to provide controlled deceleration of the motor. This is particularly important when the motor is a DC traction motor utilized in an electric vehicle such as for example, a fork lift truck in which loads may be carried in an elevated position and in which severe deceleration or braking may result in spillage of the load. In order to provide uniform braking action of the electric motor in the plugging mode, control systems are normally provided with a plugging current limit circuit which establishes a plugging current limit usually below the normal running current limit in order to provide a controlled amount of braking. In the prior art controls, circuitry for sensing the onset and duration of plugging has generally been responsive to plugging current flowing through a plugging diode which bypasses the armature of the motor. These prior art systems, however, suffer from the disadvantage that the plugging diode is thermally sensitive and has characteristics which change with operating temperature thereby requiring frequent adjustment of the plug sensing circuitry in order to maintain a controlled degree of plugging. In addition, the circuitry which is generally utilized to monitor plugging current becomes relatively complex and may itself by subject to thermal instability. In addition, there are occassions when plugging current may be sensed in the plugging diode and yet at the same time it is desired to override the plugging current limit circuitry in order to be able to provide full power to the traction motor. Such an occassion may occur, for example, in a fork lift truck which is plugging down a ramp and it is desired to drive the truck up the ramp. Under such conditions, it is necessary to provide some means for overriding the plugging current limit in order to allow the vehicle to be driven up the ramp.
It is an object of the present invention to provide an improved method and apparatus for sensing plugging of a DC electric motor.
It is a further object of the present invention to provide an improved method and apparatus in which the plugging current limit may be overriden.